1. Field of the Invention
The present invention relates generally to semiconductor memory and method of manufacturing the same and particularly to via-less memory structures and method of manufacturing same.
2. Description of the Prior Art
Magnetic random access memory (MRAM) is rapidly gaining popularity as its use in replacing conventional memory is showing promise. MRAM memory element consists of a top electrode connecting to a bit line, a magnetic tunnel junction (MTJ), and a bottom electrode connecting to a control device such as a FET transistor or a diode. The MTJ further includes a free layer, a pinned layer and a barrier layer. Manufacturing of the MRAM has offered challenges.
The size of the memory element has been reduced to minimize the memory cell area. It has become smaller than the size of the via, the latter being used to form a connection to the MTJ and to an upper metal line serving as a bit line. A side wall of the top electrode is exposed during the via hole etching. When the bottom of the via hole reaches the barrier layer, the pinned layer and the free layer are shorted together in the process of filling the via with metal. Conversely, the shallow via hole often makes no contact with the top electrode because during the via etching process, the bottom of the via does not reach the top electrode of the MTJ resulting in an undesirably open connection between the MTJ and the upper metal line. The process margin for etching is therefore limited. Also, open and short scenarios are highly dependent on the thickness of the top electrode.
Efforts have been used to overcome this problem by using a thick top electrode formed on top of the MTJ but this presents a challenge for fine patterning including photo and etch processes. Thick photo resist and/or thick hard mask is required to make a pattern in the top electrode, which does not meet the requirements for fine patterning since fine patterning requires thin photoresist and hard mask.
Further, there are multiple steps employed in building an MTJ with a separate step needed for making the via, which adds to manufacturing time and expense.
Thus, the need arises for manufacturing of magnetic memory more reliably and expeditiously.